Liquid crystal displays (LCDs) continue to improve in cost and performance, becoming a preferred display type for many computer, instrumentation, and entertainment applications. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light providing surface positioned behind the LCD for directing light outwards, towards the LCD. The challenge of providing a suitable backlight apparatus having brightness that is sufficiently uniform while remaining compact and low cost has been addressed following one of two basic approaches. In the first approach, a light-providing surface is used to provide a highly scattered, essentially Lambertian light distribution, having an essentially constant luminance over a broad range of angles. Following this first approach, with the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution in order to provide a more collimated illumination. Among proposed solutions for brightness enhancement films are those described in U.S. Pat. No. 5,592,332 (Nishio et al.); U.S. Pat. No. 6,111,696 (Allen et al); and U.S. Pat. No. 6,280,063 (Fong et al.), for example. Solutions such as the brightness enhancement film (BEF) described in patents cited above provide some measure of increased brightness over wide viewing angles. However, overall contrast, even with a BEF, remains relatively poor.
A second approach to providing backlight illumination employs a light guiding plate (LGP) that accepts incident light from a lamp or other light source disposed at the side and guides this light internally using Total Internal Reflection (TIR) so that light is emitted from the LGP over a narrow range of angles. The output light from the LGP is typically at a fairly steep angle with respect to normal, such as 70 degrees or more. With this second approach, a turning film, one type of light redirecting article, is then used to redirect the emitted light output from the LGP toward normal. Directional turning films, broadly termed light-redirecting articles or light-redirecting films, such as that provided with the HSOT (Highly Scattering Optical Transmission) light guide panel available from Clarex, Inc., Baldwin, N.Y., provide an improved solution for providing a uniform backlight of this type, without the need for diffusion films or for dot printing in manufacture. HSOT light guide panels and other types of directional turning films use arrays of prism structures, in various combinations, to redirect light from a light guiding plate toward normal, or toward some other suitable target angle that is typically near normal relative to the two-dimensional surface. As one example, U.S. Pat. No. 6,746,130 (Ohkawa) describes a light control sheet that acts as a turning film for LGP illumination.
Referring to FIG. 1, the overall function of a light guiding plate 10 in a display apparatus 100 is shown. Light from a light source 12 is incident at an input surface 18 and passes into light guiding plate 10, which is typically wedge-shaped as shown. The light propagates within light guiding plate 10 until Total Internal Reflection (TIR) conditions are frustrated and then, possibly reflected from a reflective surface 142, exits light guiding plate at an output surface 16. This light then goes to a turning film 122 and is directed to illuminate a light-gating device 120 such as an LCD or other type of spatial light modulator or other two-dimensional backlit component that modulates the light. For optimized viewing under most conditions, the emitted light should be provided over a range of relatively narrow angles about a normal N. A polarizer 124 is necessarily disposed in the illumination path in order to provide light-gating device 120 with suitably polarized light for modulation. However, since light after passing through turning film 122 is essentially unpolarized, or has at most some small degree of polarization, the polarizer 124 must absorb about half of the light. In order to overcome this problem, a reflective polarizer 125 is often provided between absorptive polarizer 124 and turning film 122.
One type of reflective polarizer is disclosed in U.S. Pat. Nos. 5,982,540 and 6,172,809 entitled “Surface light source device with polarization function” to Koike et al. The Koike et al. '540 and '809 disclosures show a surface light source device that has a light guiding plate, one or more polarization separating plates, a light direction modifier (essentially a turning film), and a polarization converter. The polarization separating plate is a type of reflective polarizer 125. The polarization separating plate described in the Koike et al. '540 disclosure utilizes Brewster's angle for separating S- and P-polarized components of the illumination. While this approach provides some polarization of the light, however, it merely provides one type of substitute for more conventional reflective polarizing films. This solution still requires the additional use of separate polarizer film or film(s). Moreover, the approach of the Koike et al. '540 and '809 disclosures requires that the index of refraction n of the material used for the polarization separating plate be within a narrow range, based on the incident angle of light from the light guiding plate.
Clearly, there would be advantages to reducing the overall number of components needed to provide polarized illumination without compromising image quality and performance. With this goal in mind, there have been a number of solutions proposed for simplifying the structure of polarizer 125 or eliminating this component as a separate unit by combining functions. In an attempt to combine functions, U.S. Pat. No. 6,027,220 entitled “Surface Light Source Device Outputting Polarized Frontal Illumination Light” to Arai discloses a surface light source device capable of producing illumination that is at least partially polarized. As the Arai '220 disclosure shows, there is inherently some polarization of light that emerges from light guiding plate 10 (FIG. 1). In addition, there is further polarization of this light inherently performed by the turning film. In a configuration that employs a pair of turning films, there can be even further slight gains in polarization. Following the approach of the Arai '220 disclosure, a surface light source can be designed that provides some degree of polarization simply by using suitable materials for each turning film and matching these materials, according to their index of refraction n, to the angle of inclination of light from the light guiding plate. While this approach has merit for providing some measure of polarization, however, there are practical limits to how much improvement can be gained based on simply specifying an index of refraction n. Moreover, embodiments utilizing multiple turning films add cost, thickness, and complexity to the illumination system design.
In yet another approach, U.S. Pat. No. 6,079,841 entitled “Apparatus for Increasing a Polarization Component, Light Guide Unit, Liquid Crystal Display and Polarization Method” to Suzuki, provides a light guiding plate that is itself designed to deliver polarized light. The Suzuki '841 light guiding plate utilizes a stack of light guides laminated together and oriented to provide Brewster's angle conditioning of the light to achieve a preferred polarization state. While this method has the advantage of incorporating polarization components within the light guide itself, there are disadvantages to this type of approach. The complexity of the light guide plate an the added requirement for a half-wave or quarter-wave plate and reflector negates the advantage gained by eliminating the polarizer as a separate component in the illumination path.
Thus, it can be seen that, while there have been attempts to provide polarized illumination by incorporating the polarization function with other components, these attempts have not provided flexible, less costly, and more effective solutions. There is, then, a need for a low cost turning film solution that provides polarized illumination with a reduced number of components.